Mounting of shelf boards on shelf supporting crossbars leads to problems in element-structured, free-standing shelf assemblies which are held together by structural elements, i.e. bearing crossbars and shelf boards and which can be extended into larger shelf assemblies. Coupling of shelf boards together for holding the assembly intact creates further problems. For example, in wood construction shelves, the coupling elements comprise screws, consoles and the like metal fasteners. When a shelf assembly is to be made of purely natural material, e.g. wood, the use of metal fasteners is out of the question. A further drawback involved in the use of metal fasteners is that the fitting, mounting and usually also the use thereof requires special tools. Metal fasteners may cause trouble when used e.g. in display racks of clothing stores since thinner clothes easily grab the generally sharp edges of metal fasteners and thus tear themselves easily. In terms of aesthetics, metal fasteners are not genereally desirable.